Various processes are known for the production of higher linear alpha olefins (for example D. Vogt, Oligomerisation of ethylene to higher α-olefins in Applied Homogeneous Catalysis with Organometallic Compounds Ed. B. Cornils, W. A. Herrmann Vol. 1, Ch. 2.3.1.3, page 245, VCH 1996). These commercial processes afford either a Poisson or Schulz-Flory oligomer product distribution.
In order to obtain a Poisson distribution, no chain termination must take place during oligomerisation.
However, in contrast, in a Schulz-Flory process, chain termination does occur and is independent from chain length. The Ni-catalysed ethylene oligomerisation step of the Shell Higher Olefins Process (SHOP) is a typical example of a Schulz-Flory process.
In a Schulz-Flory process, a wide range of oligomers are typically made in which the fraction of each olefin can be determined by calculation on the basis of the so-called K-factor. The K-factor, which is indicative of the relative proportions of the product olefins, is the molar ratio of [Cn+2]/[Cn] calculated from the slope of the graph of log [Cn mol %] versus n, where n is the number of carbon atoms in a particular product olefin. The K-factor is by definition the same for each n. By ligand variation and adjustment of reaction parameters, the K-factor can be adjusted to higher or lower values. In this way, the process can be operated to produce a product slate with an optimised economic benefit.
Since demand for the C6-C18 fraction is much higher than for the C>20 fraction, processes are geared to produce the lower carbon number olefins. However, the formation of the higher carbon number olefins is inevitable, and, without further processing, the formation of these products is detrimental to the profitability of the process. To reduce the negative impact of the higher carbon number olefins and of the low value C4 fraction, additional technology has been developed to reprocess these streams and convert them into more valuable chemicals such as internal C6-C18 olefins, as is practised in the Shell Higher Olefins Process.
However, this technology is expensive both from an investment and operational point of view and consequently adds additional cost. Therefore considerable effort is directed to keep the production of the higher carbon numbered olefins to the absolute minimum, i.e. not more than inherently associated with the Schulz-Flory K-factor.
WO-A-99/12981 describes catalyst systems for the polymerisation of 1-olefins, in particular ethylene, which contain nitrogen-containing transition metal compounds comprising a skeletal unit of formula (B),
wherein M is Fe [II], Fe [III], Ru [II], Ru [III] or Ru [IV]; x represents an atom or group covalently or ionically bonded to the transition metal M; T is the oxidation state of the transition metal M and b is the valency of the atom or group X; R1, R2, R3, R4 and R6 are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl, R5 and R7 are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl.
WO-A-00/50470 discloses catalyst compositions that it is said may be used in the polymerisation or oligomerisation of olefins.
Said catalyst compositions comprise a metal complex ligated by a monodentate, bidentate, tridentate or tetradentate ligand, wherein at least one of the donor atoms of the ligand is a nitrogen atom substituted by a 1-pyrrolyl or substituted 1-pyrrolyl group; wherein the remaining donor atoms of the ligand are selected from the group consisting of C, N, P, As, O, S and Se; and wherein said metal in said metal complex is selected from the group consisting of Se, Ta, Ti, Zr, Hf, V, Nb, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rn, Ir, Ni, Cu, Pd, Pt, Al and Ga.
Ligand, h1, is said to be one of a number of neutral tridentate ligands that may be used in preferred catalyst compositions according to WO-A-00/50470:—

In particular, a number of symmetrical bis-pyrrolyl imine ligands, h16-h17, h19-h25, h28, h30 and h32 are specifically disclosed. In addition, h29, h31 and h33 are mixed bis-pyrrolyl imine ligands.
Examples 59, 60 and 62 of WO-A-00/50470 demonstrate the polymerisation of ethylene in the presence of an iron-based catalyst composition comprising symmetrical bis-pyrrolyl imine ligand h16.
Similarly, Example 74 concerns the polymerisation of ethylene in the presence of an iron-based catalyst composition comprising symmetrical bis-pyrrolyl imine ligand h17.
Further examples of ethylene polymerisation in WO-A-00/50470, employing some of the afore-mentioned ligands in iron-based catalyst compositions, include Examples 99-125; 128, 131, 133 and 135 therein.
As exemplified in Example 4 herein, the lower homologue of ligand h16 of WO-A-00/50470, that is to say, ligand (11), gives rise to an iron-based bis-pyrrolyl imine catalyst composition which gives little, if any, ethylene conversion.